This set of Structural Biology Multiple Choice Questions & Answers (MCQs) focuses on “Domain Structures of Proteins”.
1. Motifs combine to form ___________
a) Supersecondary structures
b) Domains
c) Quaternary structures
d) Tertiary structures
View Answer
Explanation: Several motifs like beta-hairpin, helix-loop-helix, helix-turn-helix, beta-alpha-beta combine to form a domain by forming bonds between them.
2. What is the domain?
a) Polypeptide chain that can form a tertiary structure with the assistance
b) Polypeptide chain that can form a quaternary structure independently
c) Polypeptide chain that can form a quaternary structure with the assistance
d) Polypeptide chain that can form a tertiary structure independently
View Answer
Explanation: Domains are the large chains of polypeptides that can fold without any assistance into tertiary structures which are energetically stable.
3. Domains are also the units of _________
a) Division
b) Secondary structure
c) Functions
d) Folding
View Answer
Explanation: A particular protein will be having several different domains. Each domain will have a specific function. Hence, a protein can perform a particular task by the coordination of different domains.
4. It is found that there will be no fundamental distinction between a domain and a __________ of a protein.
a) Motif
b) Tertiary structure
c) Subunit
d) Quaternary structure
View Answer
Explanation: In proteins, a subunit refers to a functional unit. And as we know, the domain is an actual unit responsible for the function of a protein. They are interchangeably used to describe a functional unit.
5. In plants, a process for the synthesis of fatty acids requires the activation of 7 different biological reactions involving 7 different polypeptides. Therefore, what is the number of proteins required for the synthesis of the fatty acids in mammals?
a) 9
b) 7
c) 3
d) 1
View Answer
Explanation: For the process of synthesis of fatty acids in mammals require only one protein even though it takes 7 polypeptides in plants. The reason for this is that the protein in mammals for the synthesis of fatty acids are made of 7 domains each performing the same functions as 7 separate polypeptides.
6. A beta-hairpin motif is coded from a gene having 297 base pairs. Two such beta-hairpin motifs are assembled to form a domain. Calculate the total number of amino acids present in the domain.
a) 297
b) 99
c) 198
d) 594
View Answer
Explanation: An amino acid is coded by 3 base pairs in a gene. It is given that there are 297 base pairs in the gene coding for beta-hairpin motif. Hence the total number of amino acids in beta-hairpin motif is 297/3 = 99. Thus, a domain formed from the assembly of 2 beta-hairpin motifs have 99×2 = 198 amino acids.
7. Which one of the following is not a type of domain classified by M. Levitt and C. Chothia?
a) Alpha domain
b) Alpha/beta domain
c) Beta domain
d) Beta/alpha domain
View Answer
Explanation: Two scientists named M. Levitt and C. Chothia identified and classified the different types of domains based on the motifs building those domains in MRC molecular biology laboratory.
8. Which is the fourth type of domain that does not include in Levitt and Chothia’s scheme?
a) Alpha/beta
b) Beta/alpha
c) Alpha+beta
d) Beta+alpha
View Answer
Explanation: Alpha+beta is the unique group with less number of domains under this category. These domains are made up of several alpha helices with a single anti-parallel beta strand packed against the several alpha helices.
9. The alpha/beta is a combination of ________ motifs.
a) Alpha helices
b) Anti-parallel beta strands
c) Beta-alpha-beta
d) Helix-turn-helix and beta-hairpin motif
View Answer
Explanation: The alpha/beta domains are made up of beta-alpha-beta motifs. Hence, the domain contains a huge number of parallel beta strands with some alpha helices between them. These helices connect parallel beta pleats to form alpha/beta domains.
10. Myoglobin is made up of _______ domains.
a) Alpha
b) Beta
c) Alpha/beta
d) Alpha+beta
View Answer
Explanation: Myoglobin is a globular protein with several alpha domains. Each domain has a huge number of alpha helices joined by loops to each other. Hence, they constitute a hydrophobic core and hydrophilic surface making it a soluble protein.
11. What are coiled-coils?
a) Supercoiling of beta pleats
b) Supercoiling of alpha helices
c) Supercoiling of motifs
d) Combination of domains
View Answer
Explanation: The coiled-coil is the combination of alpha helices. In coiled-coil helices, a chain of amino acid residues in the form of an alpha helix which are mostly hydrophobic wind with each other forming supercoils of proteins.
12. Which of the following domain does not contain helices?
a) Alpha domain
b) Beta domain
c) Alpha/beta domain
d) Alpha+beta
View Answer
Explanation: As the name suggests, alpha domains contain alpha helices. And alpha/beta domains contain several beta-alpha-beta motifs which also contains alpha helices linking parallel beta sheets. Alpha+beta domains also contain alpha helices but in less number. But the beta domain does not contain alpha helices. Hence, the answer is the beta domain.
13. In four-helix structure, the polar groups are buried inside in the core.
a) True
b) False
View Answer
Explanation: The four-helix is built up of four alpha helices. These four alpha helices are arranged in such a way that their helix axes are almost parallel to each other. The polar side chains of amino acids of alpha helices building the four-helix structure are exposed out to solvent and the non-polar side chains are buried inside the alpha helices.
14. Domains are always continuous.
a) True
b) False
View Answer
Explanation: It has been found that all the domains are not continuous. About 1/4th of the proteins are known to have a discontinuous arrangement of polypeptides that form domains.
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